media: atomisp: Use new atomisp_flush_video_pipe() helper in atomisp_streamoff()

Use the new atomisp_flush_video_pipe() helper instead of open-coding it.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>